A person recognizes an acoustic space on the basis of a loudness difference and a time difference in the sound inputted to two ears. A method of expressing an acoustic space by using this fact and employing left and right two speakers, that is, stereo recording, is generally used. In this acoustic system, a method called panning is also used that a mutual difference is imparted to the sound pressures of the sound emitted from the left and the right speakers, so as to cause a person to feel as if the sound were generated at a position between the speakers. Another method is also used that a time difference in the sound reaching microphones arranged at intervals is used so that a similar effect is achieved.
Nevertheless, the sound emitted from the left and the right speakers attenuates depending on the distance and a difference arises in the propagation time of the sound from the speakers. This results in a sound pressure difference and a time difference in the sound from the left and the right speakers depending on the position of the listener. Thus, the anticipated panning effect is achieved only on the center line located at equal distances from the two speakers arranged left and right. That is, a listener who listens the sound at any other position feels as the sound is emitted from the speaker located closer to the listener.
As a method for resolving the problem, for example, A. J. Berkhout, D. de Vries, and P. Vogel, “Acoustic control by wave field synthesis” (The Netherlands), 93(5)-th Print, Journal of the Acoustical Society of America (J. Acoust. Soc.), May 1993, pp. 2764-2778 describes an acoustic system employing a WFS (Wave Field Synthesis) technique and synthesizing a wavefront of sound by using an array speaker. In the WFS technique, an array speaker is employed that is constructed from a plurality of speakers arranged in one row. Then, the sound waves emitted from the respective speakers are superposed with each other so that a wavefront of the sound is synthesized such that a person should feel as if a sound source were located at the center point of the wavefront of the sound. The WFS technique reproduces the wavefront itself of the sound and hence, in a larger region, causes a person to feel as if the sound source were located at an anticipated position. Here, the center point of the wavefront of the sound generated by the array speaker is referred to as a virtual sound source.
In the plurality of speakers constituting the array speaker, a smaller value for the mutual arrangement intervals permits more satisfactory reproducibility of the wavefront of sound at higher frequencies. On the other hand, a larger value for the arrangement width of the array speaker expands the size of a space where the wavefront of sound is satisfactorily reproduced. Physically, the diameter of each speaker contained in the array speaker is smaller than or equal to each arrangement interval of the speakers. Thus, in order that an acoustic system satisfactorily reproducing the wavefront of sound at high frequencies should be realized, the diameter of each speaker need be reduced. On the other hand, the diameter of each speaker is restricted also by installation requirements in many cases. In particular, in a case that a speaker is to be incorporated into a television set, in order that the presence of the speaker should not extremely be apparent, an approach is often taken that the height or the width of the speaker is reduced and so is the area and that a speaker having a small diameter corresponding to the shorter side is employed. In another reason, for the purpose of cost reduction in the entire acoustic system including a signal processing part, in some cases, a large value is adopted for the arrangement intervals of speakers and, at the same time, speakers having relatively small diameters are selected.
Nevertheless, in a speaker having a small diameter, since its diaphragm area is small, the sound pressure obtainable with a finite diaphragm amplitude is limited. Thus, in such a speaker having a small diameter, problems on the acoustic characteristics arise, for example, that a high sound pressure is not generally achieved and that the reproduction frequency band is insufficient on the low pitch side. Further, as a result of the structure, the size of the diaphragm amplitude itself is smaller than that of a speaker having a large diameter. Although enlargement of the diaphragm area is effective for obtaining a higher sound pressure, in a case that any restriction prevents adoption of a speaker having a large diameter, a technique is employed that for the purpose of obtaining a sufficient sound pressure, a plurality of speakers are connected in parallel or in series and then the plurality of speakers are driven with the same signal so that the diaphragm area is equivalently increased. Nevertheless, when a plurality of speakers are driven with the same signal, the emitted waves from the speakers interfere with each other and hence high directivity is caused in the sound waves. In an array speaker, the sound waves emitted from the respective speakers are superposed with each other so that a wavefront of the sound is synthesized. Thus, it is appropriate that the emission characteristics of each speaker is of non-directivity. Accordingly, driving a plurality of speakers with the same signal causes disturbance in the wavefronts of the sound and hence prevents achievement of the original performance of the array speaker. On the other hand, in order that the reproduction frequency band on the low pitch side should be expanded, enlargement of the diameter of the speaker is effective. Nevertheless, this approach causes similar problems and hence is not practical in many cases.
Further, in the case of an array speaker, when the intervals of a plurality of speakers constituting the array speaker are large, a problem arises that a listener close to the array speaker feels that a sound source is located at the position of a speaker near the listener. This is because the curvature of each wavefront emitted actually is smaller than that of the wavefront to be reproduced. In the WFS, wavefronts generated by the respective speakers are superposed with each other so that a wavefront is reproduced. Then, when the speaker density is insufficient for wavefront superposition, a smooth wavefront is not reproduced. In other words, in order that a smooth wavefront should be reproduced, the intervals of the speakers need be reduced so that a sufficient speaker density need be ensured. An employable approach for achieving this is that speakers are added for complementing the wavefronts emitted from the original speakers.
In conclusion, in order that an acoustic system employing an array speaker should reproduce sound at higher frequencies in a larger region, speakers having small diameters need be arranged at intervals as narrow as possible and in a number as large as possible and then these speakers need be controlled.
However, on the other hand, in order that a high sound pressure should be obtained in lower frequency sound, speakers having diameters as large as possible need be incorporated in a number as large as possible into the acoustic system.
As a method for resolving such problems caused by the relation between the size of the diameter of the speaker and the width of the frequency band, for example, Japanese Patent Application Laid-Open No. 2006-67301 describes an apparatus in which an array speaker is constructed by combining a plurality of speakers having mutually different diameters for each frequency band so that the sound frequency band is broadened.